Solved for me!!!
I use a Mac and am using Safari as a browser. I thought there could be an issue with Safari as I see there or other problems lately with that browser. So ... I tried Chrome. Same same. Could not update the Path nor set the Update Channel. Then I thought wonder if it is a Macintosh Issue. Tried the settings change on my windows machine.
VIOLA!! I logged in with Chrome on my windows machine and the WHMCS Update looked different. Then clicked "Configure Update Settings" and was able to save the settings!!!
Changing the settings works for me in a Windows Chrome browser but not a Mac Safari, not a Mac Chrome browser.